BOSTON, Massachusetts (CNN) -- It all started with the flush of an automatic toilet. The terrifying sound marked the beginning of a two-year nightmare for Sarah Teres as she desperately tried to potty train her daughter Molly.

Molly graduated from Potty School four days before her brother, Archer, was born.

"It was awful" Teres said. "We tried everything including bribery and threats." Teres, the mother of three from Andover, Massachusetts, hoped her middle child would be toilet trained by the time she was 2˝.

Two years later, the girl was still in diapers, refusing to use the bathroom. "I was going crazy," Teres admitted. "She wouldn't poop. She would hold it for days."

At wits end, Teres enrolled Molly in the Toilet Training School at Children's Hospital Boston.

"By the time the children come in with their families, it has become a power struggle," explained Dr. Alison Schonwald, a pediatrician who supervises the "poop school," as it's affectionately called by staffers. "The kids kind of dig in their heels and put a line in the sand." Health Minute: Watch more on the perils of potty training »

The American Academy of Pediatrics reports that most children show signs they're ready to potty train between 18 and 24 months. Doctors suggest a child may be ready to start trying if he's staying dry for at least two hours at a time during the day, walking to and from the bathroom, asking for a diaper change and asking to use the potty.

For some kids, the toilet training process can take more than a year, or longer.

The six-week program at Children's Hospital is one of a handful around the country. Kimberly Dunn, a pediatric nurse practitioner, has worked with some of the 450 young graduates over the years.

She said most of the kids admit they are afraid to use a toilet. "Oftentimes, the parents come in and they want to know why they're afraid," Dunn said. "You could ask the kids until they're blue in the face and you hardly ever find out why."

Dunn meets with a half-dozen children once a week. She uses books, music and art to help the students overcome their fear of using the toilet. She helps them set small, realistic goals. For instance, she said, week one involved just sitting on the toilet for five minutes. She encourages positive reinforcement and simple rewards such as extra playtime with Mom or Dad.

Ready for the potty?

Here are signs that your child is ready for toilet training:

Stays dry at least 2 hours at a time during the day

Bowel movements become regular

Child can follow simple instructions

Child can walk to and from bathroom and undress

Child seems uncomfortable with soiled diapers

Child asks to use the toilet

Child asks to wear grown-up underwear

Source: American Academy of Pediatrics

While Dunn works on the kids, psychologist Elaine Leclair, an instructor at the Harvard School of Medicine, offers frank advice to the parents in a separate room.

"I just say, 'Step back.' They hate to hear me say this, but I say whatever you're doing now is not working. You really need to try something different," Leclair said.

She said many parents come to the sessions angry and anxious. "They come in feeling extremely discouraged, very isolated thinking they are the only ones in the world who have this problem."

Teres acknowledged that's how she felt. "Imagine my surprise to find out there were thousands of kids who had this problem." After years of hiding her daughter's toilet training troubles from family and friends, the group parenting sessions allowed Teres to open up about her frustrations.

"It was like going to Betty Ford," joked Teres, who felt she had exhausted all her other options.

A majority of the children who attend the Children's Hospital class are dealing with constipation issues often caused by delayed toilet training.

Teres learned that her daughter had a medical condition called encopresis. Experts call it a symptom of chronic constipation and say it occurs when a child resists having a bowel movement.

Youngsters like Molly are sometimes given laxatives or other medications to help encourage them to go.

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Schonwald, the author of "The Pocket Idiot's Guide to Potty Training," doesn't want parents to be discouraged during the process. "No one goes to college in diapers, right? Everyone will get through this time, as awful as it might feel if you're struggling."

She suggested achieving success during potty training has to do with the approach. "There are three things you cannot make a child do: eat, sleep or poop. We find that by decreasing the pressure and expectations that children feel more confident."

Schonwald reminds parents that toilet training "is not a chore, so keeping it positive from the very beginning is the most important thing." If a child isn't using the toilet by the age of 4, she recommends talking with a pediatrician. Schonwald also said setbacks during potty training are normal. "All developmental skills come in spurts with periods of regression."

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After a setback, Schonwald advises parents to give their child positive messages that they can succeed by saying, "So you had some accidents today. We'll try again tomorrow."

Four months after graduating from "poop school," Teres is relieved to report that Molly is fully toilet trained. "You would never know this was an issue," said Teres. "She even used an automatic flush toilet the other day. I waited to see her reaction and she was OK."E-mail to a friend

Judy Fortin is a correspondent with CNN Medical News. Linda Ciampa of Accent Health contributed to this report.